Thermochemistry

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Thermochemistry

• Study of heat in chemical reactions.
• Heat will travel between a system and its
  surroundings
• Heat is measured in Joules (J)
• Metric ? English BTU (British Thermal Unit)
• 1 BTU 1.06 kJ

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Calculating Heat

• The heat that transfers between substances can be
  calculated with the following equation
• q (s) (m) (?T)

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Calculating Heat

• q amount of heat (energy) transferred
• s specific heat
• m mass (use density to change from mL to g!)
• ?T change in temperature

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Calculating Heat

• Specific Heat is the quantity of heat needed to
  raise one gram of a substance 1.0 C.
• Each specific substance has its own specific
  heat. (pg. 394)

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Round to 4.18
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PG
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Calculating Heat

• Example
• How much heat is needed to warm a 200.0 mL cup of
  coffee (H2O) from 20.0 C to 75.0 C?

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Calculating Heat

• 45980 J, but only 3 significant digits so.
• 4.60 x 104 J or 46.0 kJ

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Calculating Heat
What is the specific heat of lead?
28.8 C
20.0 C
100.0 C
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1st Step - Calculate q for the H2O
20.0 C
q 1840 J
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2nd Step - Calculate s for Lead
Energy that water absorbed in step one was
supplied by the metal!
100.0 C
q (150.0 g)(s)(71.2oC)
1840 J
s 0.172 J/(g x C) for Lead